The only reason I didn't use Mustache was because it was unnecessarily complicated for my very simple use case. I just needed to effectively include blocks from other files in order to be able to share boilerplate between pages, plus having basic variable substitution for things like the page name. I even considered using the C preprocessor instead!

But I definitely wouldn't recommend using m4 for anything beyond a quick hack, unless you're a m4 wizard (or willing to become one) and you trust that whoever is going to work on the project is going to be one too. It's definitely clunky and not very user friendly.